My invention relates to digitally controlled high power transistorized inverter-rectifier combinations and, more particularly, to a protective circuit for high power transistorized inverter-rectifier combinations containing digital type driving means for controlling alternate operation of the transistors.
Inverter-rectifiers are a class of known electrical apparatus having the function of converting an input DC voltage to a DC voltage of a different level using elements which develop from the DC voltage applied AC voltages or currents and transforms such AC to a different AC voltage. This inversion process is followed by rectification to produce the DC voltage level required and, typically, the apparatus includes regulating means to stabilize the output DC voltage at the desired level. Conventional inverter-rectifiers are transistorized and in those designs at least a pair of transistors are coupled to a transformer either through a center tapped primary winding or through individual primary windings. Each of the transistors alternately conducts current for an interval through the associated transformer winding or winding portion under control of a drive voltage pulse applied alternately to the transistors, and the transformer windings are arranged so that current which flows under control of one transistor is electrically "poled" oppositely to the direction of current through the winding portion conducted by the second transistor. Hence, by alternately turning each transistor on and off, an alternating magnetic field is created within the magnetic transformer core to induce an AC voltage in a secondary winding.
The high power inverter-rectifier combinations employ digital type semiconductor means to provide drive pulses alternately to the inverter transistors. One inverter combination with which I am most familiar is the subject of a prior invention described in a co-pending application for patent, Ser. No. 686,797, filed May 17, 1976, and now U.S. Pat. 4,017,784, for a DC to DC converter, assigned to the assignee of this application, the disclosure of which is by reference specifically incorporated herein in its entirety. The cited application discloses a digital type semiconductor circuit employing various monostable multivibrators, Nand gates and the like, which are used to provide pulses alternately to inverter transistors Q.sub.1 and Q.sub.2 or to two pairs of transistors Q.sub.11 and Q.sub.41 and Q.sub.21 and Q.sub.31, each pair of which conducts current alternately.
In the operation of the inverter the transistors conduct current alternately under control of digital circuits and it is abnormal if such transistors conduct current simultaneously for any interval, however short. Undoubtedly, in some of such prior art inverters which I characterize as low power, it is conceivable that transistor damage does not result if the transistors intended for alternate current conduction do so simultaneously. However, in high power inverter-rectifiers, for example in the range of 100 watts and above, simultaneous operation of the inverter transistors is quite likely to cause transistor damage. The burdens associated with electrical equipment failure of that type is evident, particularly where the inverter-rectifier combination is part of an airborne military electronic system where untimely failure can be critical and, in better circumstances, the replacement of high power transistors is very expensive.
In theory, such digital control circuits prevent both inverter transistors from simultaneously being placed in the current conducting or "on" condition. In practice, such circuits appear somewhat reliable. Thus, I was confronted with inverter transistor failure in the inverter described in said cited patent application, a failure which had no apparent cause. Although the inverter functioned properly in one power supply, it failed in use with a second seemingly identical power supply. And a check of the digital control circuits in the failed apparatus showed them to function as designed to provide voltage pulses in an alternate manner to each inverter transistor. The speculation is made that electronic noise of a fleeting character was present for a short moment, such as that which could be generated by power supply line transients or interruptions in or arcing of the electrical load supplied by the inverter-rectifier combination which, although a fleeting and difficult to detect phenomenon, could be sufficient to cause malfunction of digital type semiconductor control circuits which are known have limited electronic noise immunity, I deduced that a malfunction, thus, in the digital drive circuits could result in driving both inverter transistors into the current conducting condition simultaneously for a moment and that overlapping operation was sufficient to destroy at least one transistor, leading to the failures of mysterious and undefined origin encountered in practice.
A major purpose of the invention is to improve the reliability and life of high power inverter rectifier combinations.
The present invention accordingly minimizes simultaneous or overlapping operation of inverter transistors intended for alternate operation arising from malfunction of the inverter's associated digital control circuits, such as caused by ambient electronic noise. A further object of the invention is to enhance the reliability and life of transistorized high power inverter-rectifier combinations. And an ancillary object of the invention is to provide a protection circuit for inverter transistors which is versatile in application and relatively immune to the kind of transient voltages that adversely affects electrical inverter apparatus digital control circuits.